Induction of anti-PF4/heparin antibodies after arthroplasty for rheumatic diseases

Antibodies against Platelet Factor 4 and Their Associated Pathologies: From HIT/HITT to Spontaneous HIT-Like Syndrome, to COVID-19, to VITT/TTS

Antibodies against platelet factor 4 (PF4), a protein released from alpha-granules of activated platelets, may cause a number of pathophysiological conditions. The most commonly known is heparin-induced thrombocytopenia (HIT), which develops in a small proportion of people treated with the anticoagulant drug heparin. Notably, PF4 binds with high affinity to heparin, and in HIT, complexes of PF4/H may, in a small proportion of susceptible patients, trigger the development of anti-PF4 antibodies and subsequent platelet activation and aggregation, ultimately leading to the development of pathological thrombosis at sites of vessel occlusion. Of more modern interest, antibodies against PF4 may also arise in patients with COVID-19 (Coronavirus Disease 2019) or in patients who have been vaccinated against COVID-19, especially in recipients of adenovirus-based vaccines. For this latter group of patients, the terms VITT (vaccine-induced [immune] thrombotic thrombocytopenia) and TTS (thrombotic thrombocytopenia syndrome) have been coined. Another category associated with this pathophysiology comprises those in whom a precipitating event is not clear; this category is referred to as ‘spontaneous HIT-like syndrome’. Despite its name, it arises as an HIT-mimicking disorder but without antecedent heparin exposure. In this narrative review, we describe the development of antibodies against PF4, and associated pathophysiology, in such conditions.

Spontaneous HIT syndrome: Knee replacement, infection, and parallels with vaccine-induced immune thrombotic thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is characterized clinically by thrombocytopenia, hypercoagulability, and increased thrombosis risk, and serologically by platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies. Heparin-"induced" acknowledges that HIT is usually triggered by a proximate immunizing exposure to heparin. However, certain non-heparin medications (pentosan polysulfate, hypersulfated chondroitin sulfate, fondaparinux) can trigger "HIT". Further, naturally-occurring polyanions (bacterial lipopolysaccharide, DNA/RNA) can interact with PF4 to recapitulate HIT antigens. Indeed, immunologic presensitization to naturally-occurring polyanions could explain why HIT more closely resembles a secondary, rather than a primary, immune response. In 2008 it was first reported that a HIT-mimicking disorder can occur without any preceding exposure to heparin or polyanionic medications. Termed "spontaneous HIT syndrome", two subtypes are recognized: (a) surgical (post-orthopedic, especially post-total knee arthroplasty, and (b) medical (usually post-infectious). Recently, COVID-19 adenoviral vector vaccination has been associated with a thrombotic thrombocytopenic disorder associated with positive PF4-dependent enzyme-immunoassays and serum-induced platelet activation that is maximal when PF4 is added. Vaccine-induced immune thrombotic thrombocytopenia (VITT) features unusual thromboses (cerebral venous thrombosis, splanchnic vein thrombosis) similar to those seen in spontaneous HIT syndrome. The emerging concept is that classic HIT reflects platelet-activating anti-PF4/heparin antibodies whereas spontaneous HIT syndrome and other atypical "autoimmune HIT" presentations (delayed-onset HIT, persisting HIT, heparin "flush" HIT) reflect heparin-independent platelet-activating anti-PF4 antibodies-although the precise relationships between PF4 epitope targets and the clinical syndromes remain to be determined. Treatment of spontaneous HIT syndrome includes non-heparin anticoagulation (direct oral Xa inhibitors favored over direct thrombin inhibitors) and high-dose immunoglobulin.